Pulse controlled solenoid valve

ABSTRACT

A pulsed controlled solenoid flow control valve suitable for use in a closed vapor cycle air conditioning system is disclosed. A pulsewidth modulated control signal is generated for cyclically opening and closing the flow through the expansion valve. The duty cycle of the pulsed control signal determines the average flow rate through the valve. An exponential response control curve is used in conjunction with an integrator offset to obtain a single set point control operating point for all flow rates through the valve, where a given change in the second superheat of the evaporator produces the same percentage change in flow rate regardless of the flow rate.

BACKGROUND OF THE INVENTION

The present invention relates to electrically actuated solenoid fluidflow control valves. More particularly, the present invention relates toa solenoid flow control valve through which a desired fluid flow rate isdetermined by the controlled oscillatory energization of the solenoid.

A valve which meters fluid flow therethrough in accordance with flowdemand, i.e., how much volume of fluid is permitted passage through thevalve for a given period of time, typically operates in connection witha control signal developed by sensing a system condition. If the valueof the sensed condition is different than a predetermined desiredoperating point, a control signal is produced for changing the fluidflow opening of the valve to meet the changed flow demand.

A fluid flow control valve is generally designed to operate over a rangeof flow demands. Typically for such fluid flow valves, the responsecurve defining the relationship between the sensed condition and theresulting fluid flow rate through the valve is linear over thisoperating range.

For such a prior-art valve, a given change in the sensed condition at alow demand flow rate will produce a certain change in the flow ratethrough the valve. This change in flow rate relative to the operatingdemand flow rate can be expressed as a percentage change. When the valveis operating at a high demand flow rate, the same given change in thesensed condition still produces the same amount change in the flow rate.This amount of change, when expressed as a percentage of the flow rateat the higher operating demand condition, will be less than it was forthe lower operating condition. Thus, to effect the same percentagechange in the flow rate at the higher demand level, a greater change inthe sensed condition must occur. This greater change in the sensedcondition to effect the proper change in flow rate represents adisadvantage in these prior-art valves. A control system is more stablewhen the system can be controlled to the desired operating pointresponsive to small changes in the sensed condition.

An additional disadvantage in these prior-art valves is that the setpoints or operating points for the sensed condition change dependingupon what the demand flow rate through the vavle happens to be. Thus,for a 30% demand condition, the operating point would be one value whilea 60% demand condition would require a second higher operating setpoint.

A further disadvantage present in these prior-art flow control valves ischaracterized by a hysteresis error between the control signal appliedto affect a flow condition and the actual flow condition which results.In an error free system, a given control signal should produce aparticular flow rate through the valve. Where hysteresis errors arepresent, changing the control signal a given amount to effect a givenchange in the flow rate as predicted by the system control transferfunction does not necessarily result in such desired change.

This hysteresis error is due to the valve's inability to achieve thedesired orifice opening because of mechanical errors, magnetic errors,etc., in the valve's components. In a closed loop control system, suchhysteresis errors will result in a continual "hunting" effect by thecontrol signal since any demand must exceed the hysteresis error beforeany actual change in the flow rate is affected, i.e., the system isessentially underdamped. Such control never actually catches up to thedemand. This hysteresis effect is the same whether the demand flow rateincrease or decreases.

Accordingly, it would be advantageous to provide a solenoid flow controlvalve which operates with essentially zero hysteresis error thereby toachieve the accurate control of the flow rate therethrough. It wouldalso be advantageous to provide a solenoid flow control valve whichcould be operated in a closed loop control system with only one setpoint regardless of the flow demand condition through the valve, with acontrol response function which produces the same percentage change inflow rate to a given sensed condition change at a high demand flow asoccurs for the same condition change at a lower demand condition.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is disclosed a solenoidflow control valve with an associated control circuit for use in aclosed vapor cycle refrigeration system. A plurality of like sensors,temperature sensors being preferred, monitor various parametersassociated with the refrigeration system, such as monitoring therespective inlet and outlet conditions of the evaporator coil. Adifferential amplifier responds to the inlet and outlet temperaturesensors to produce a signal responsive to the difference therebetween(the "superheat" of the system) to establish a normal operating setpoint for the system. The output of the differential amplifier isapplied to an integrator control circuit which produces two outputs, afirst output which is applied directly to a second amplifier, and asecond signal which is applied to an integrator. The output from theintegrator is applied to the second amplifier as an offset controlvoltage.

The output from the second amplifier is, in turn, applied to a functiongenerator which produces a control signal which eventually is applied tothe solenoid of the flow control valve. In one embodiment of the presentinvention, the function generator is a voltage-to-pulse-width converterwhich produces a pulsewidth modulated frequency signal to the solenoidvalve. The nature of the pulsewidth modulated signal either fully opensor fully closes the solenoid valve such that the duty cycle of theopen-to-close conditions determines the average flow rate through theexpansion valve. In an alternate embodiment of the invention, thefunction generator produces a control signal to the solenoid of theexpansion valve which has both a DC component and an oscillatorycomponent whose amplitude must be large enough to produce movement ofthe orifice closing mechanism to exceed the hysteresis error in thevalve. The presence of the oscillatory component causes the orificeopening of the expansion valve to continuously oscillate between twosizes. In this manner, on the average, the hysteresis effects of thesolenoid valve are averaged out so that the average size of the orificeopening between the two positions produced by the oscillatory componentof the control signal is the average orifice opening size needed for thedesired controlled flow rate.

For the embodiment wherein the function generator is avoltage-to-pulsewidth modulator, the conversion response function forsuch pulsewidth modulator produces an exponential response functionrelating the sensed difference between the outlet and the inlettemperatures and the size of the orifice opening for the solenoidexpansion valve. An exponential response function produces a greaterchange in flow rate at a higher demand condition for a given change inthe sensed condition than the same change produces at a lower demandcondition.

For such an embodiment, the integrator functions to shift the responsecurve of the voltage-to-pulsewidth converter as a function of the demandconditions on the refrigeration system so that a single set point isthereby obtained. Because of the shifting of the exponential responsecurve to obtain the same set point, a higher demand condition respondswith the same percentage change in flow rate for the same change insensed temperature difference between the outlet and the inlettemperature sensors as occurs for a lower demand condition. In oneembodiment of the invention, the integrator is an analog circuit usingoperational amplilfiers, while in an alternate embodiment, theintegrator function may be performed by a digital circuit comprised ofan up/down counter and digital-to-analog converter. The count in theup/down counter represents a control parameter which effects the flowrate through the solenoid valve, and is periodically updated as afunction of the sensed system condition.

In another aspect of the invention, an additional third temperaturesensor can be provided to monitor the condition of the liquid line fromthe condensor coil to the expansion valve. If the temperature of theliquid line sensor on the upstream side of the expansion valve is thesame, or essentially the same, as the temperature on the downstream sideof the valve, there is a strong likelihood of gas being present in thecondensor line. A comparator compares the output from the inlet andliquid line sensors to produce an output which overrides the controlsignal to the solenoid expansion valve to keep the expansion valve opensubstantially all of the time until a desired minimum temperaturedifference exists between the liquid line sensor and the inlet sensor.Such a minimum temperature difference indicates that expansion isoccurring at the expansion valve, i.e., there is liquid now in thecondensor line. This feature insures start up operation even if thetemperature difference between the inlet and outlet sensors is less thanor equal to zero. Another method to insure start up, especially when ina non-low ambient start condition, is to maintain a minimum pulsingcondition to the solenoid valve at zero degree superheat.

An additional sensor can also be provided to develop a temperaturecontrol signal to control the operation of the expansion valve dependingupon the ambient air condition. If the air temperature sensor is placedin the return air for the unit being cooled and the temperature dropsbelow a threshhold setting, the flow through the solenoid expansionvalve is throttled back to control the ambient air condition. In oneembodiment of the invention, the air temperature sensor is applied to acomparator and is compared to a voltage representative of the threshholdtemperature. The output from the comparator is used to close thesolenoid expansion valve thereby decreasing the cooling rate until theambient air condition warms up.

The inlet sensor can also be used to develop a maximum operatingtemperature signal to restrict the flow through the expansion valvethereby to avoid the possibility of damaging the compressor. The inletsensor is inputted to a comparator where it is compared to a threshholdsetting representative of a preselected maximum temperature. The outputfrom the comparator is applied to the integrator control circuit toaffect the rate at which the integrator generates an offset to thesecond amplifier. The integrator is controlled when the operatingtemperature exceeds the maximum operating temperature so as to effect adecrease in the flow rate through the solenoid expansion valve.

Liquid flood conditions can also be minimized in the outlet line fromthe evaporator coil by utilizing signals from the inlet and outletsensors. When a lack of superheat condition is detected showing theexistence of potentially damaging liquid in the line from the evaporatorcoil to the compressor, a change in the response time of the integratorcan also be affected to rapidly reduce the flow rate in the solenoidexpansion valve. This reduction in flow rate results in less fluid intothe evaporation coil until conditions warm up and provide a superheatedgaseous condition at the outlet side once again.

In another aspect of the invention, the expansion valve can becontrolled to effect communications through the fluid refrigerant byencoding information into the fluid as high frequency pressurefluctuation. In yet another aspect of the invention, a mechanical valveis disclosed whereby one of the inlet and outlet ports is operated insheer action by a closure carried by a solenoid-activated plunger. Theapplied electronics and the magnetics of the relative plunger andhousing arrangement are such that the plunger does not abruptly act whenresponding to the application of the control signal.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the invention, reference should be had tothe following detailed description of the preferred embodiment of theinvention taken in conjunction with the accompanying drawings in which:

FIG. 1 is a functional block diagram of a closed vapor cyclerefrigeration or air conditioning system incorporating anelectrically-actuated solenoid fluid flow valve and an associated valvecontrol circuit operated in accordance with the present invention;

FIG. 2 is an illustration of the control response curve for a typicalprior-art expansion valve as used in a closed vapor cycle airconditioning system;

FIG. 3A is a graphical illustration of the control response curves for aclosed vapor cycle air conditioning system operated in accordance withthe present invention;

FIG. 3B is a graphical representation of the control response curves fora closed vapor cycle air conditioning system operated in accordance withthe present invention where the present invention utilizes the linearrelationship typically found in prior-art expansion valves;

FIG. 4 is a functional block diagram of the expansion valve controlcircuit 10 illustrated in FIG. 1;

FIGS. 5A, 5B, and 5C, together comprise a detailed circuit diagram ofthe expansion valve control circuit 10 when FIG. 5C is placed to theright of FIG. 5B, and FIG. 5A is placed to the left of FIG. 5B;

FIG. 6 is a timing diagram illustration of the pulsewidth modulatedsignal from the voltage-to-pulsewidth converter 26 for the two typicaloperating conditions as illustrated in FIG. 3A;

FIG. 7 is a vertical cross-sectional view of a preferred embodiment of amechanical valve in accordance with the present invention; and

FIG. 8 is a vertical cross-sectional view of a solenoid coil and housingcombination for providing magnetic flux to the plunger shown in FIG. 7.

Similar reference numerals refer to similar parts thoughout the severaldrawings.

DETAIL DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings and first to FIG. 1, there is illustrateda functional diagram for a closed vapor cycle refrigeration or airconditioning system which incorporates an electronically-actuatedsolenoid flow control valve 38, operated in accordance with the presentinvention, and functioning as the expansion valve for the airconditioning system. The solenoid expansion valve 38 responds to itsassociated expansion valve control circuit 10 as part of a closed loopfeedback control system for regulating the cooling from the airconditioning system.

A closed vapor cycle air conditioning system generally comprises acompressor 30, a condensor 34, an expansion valve 38, and an evaporatorcoil 44, all connected in a closed series loop generally in the ordergiven. The expansion valve is the throttling or metering device whichcontrols the operation of the system, and typically, such expansionvalves are mechanical devices. The refrigerent gas is first compressedin the compressor 30. The compressed refrigerant vapor is thendischarged to a condensor coil through line 32, where it is cooled andcondensed to a liquid refrigerant into the liquid line 36. The liquidrefrigerant then flows through the expansion valve 38, expanding whileit does so. In some systems, line restrictions 71, 73, respectively infront of and behind expansion valve 38, can be incorporated to helpregulate the flow rate through the valve. Conventionally, the expansionvalve is controlled by the superheat of the return gas in the suctionline 46 at the outlet end of the evaporator coil 44. Superheat is a termof art which is generally defined as the temperature of the refrigerantvapor above the evaporated temperature of the refrigerant, both measuredat the same pressure. In closed vapor cycle air conditioning systems,the superheat of the refrigerant coolant is generally defined as thetemperature difference between the temperature of the gaseous vapor atthe outlet side of the evaporator coil 44 (T_(o)) and the temperature ofthe liquid coolant at the inlet side of the evaporator coil (T_(i)).This temperature differential is taken as an acceptable approximation ofthe true superheat for the system, particularly on evaporator coilshaving a low pressure drop from inlet to outlet.

The fluid exits from the expansion valve 38 into the inlet line 40 ofevaporator coil 44 as a two-stage mixture of liquid and gas. While thisfluid into the evaporator coil 44 contains gas bubbles, it is primarilyin liquid form. As the mixture then flows through the evaporator coil44, it is in a heat exchange relationship with the compartment or unit14 to be cooled. Air is generally blown across the evaporator coil by afan 16 to perform the heat exchange operation between the cooledevaporator coils and the air flowing thereover. Hence, heat istransferred from the compartment to the refrigerant flowing through theevaporator coil, causing the liquid to boil. In normal operations, therefrigerant in the evaporator coil 44 assumes a superheated gaseousstate by the time it exits at the outlet 46 of the evaporator coil 44.The refrigerant gas is then passed through the suction line 46 to thecompressor 30, where the cycle is again initiated with the compressionof the refrigerant gas.

In a closed loop control system, the expansion valve 38 is commonlyoperated in response to the superheat temperature (T_(sh) =T_(o) -T_(i))in the suction line 46 to the compressor. Such a control device attemptsto maintain a constant superheat condition in the fluid leaving theevaporator coil 44. Should any liquid still be included in therefrigerant gas as it enters the compressor 30, the compressor will notoperate properly and will, in time, most likely be damaged.

As the liquid refrigerant passes through the expansion valve 38 into theinlet line 40 to the evaporator coil 44, the refrigerant encounters alower pressure on the downstream side of the expansion valve than ispresent on the liquid line 36. This pressure differential causes theliquid refrigerant to boil, evaporate and thus absorb heat.

In accordance with the present invention, the expansion valve 38, shownin FIG. 1, is an electrically-actuated solenoid flow control valve whichis controlled from the expansion valve control circuit 10. Varioustemperatures within the air conditioning system are detected by theexpansion valve control circuit 10 to produce a control signal on line 7to the solenoid expansion valve 38. For example, an inlet temperaturesensor 52 is positioned proximal the downstream side of the solenoidexpansion valve 38 to sense the temperature T_(i) of the liquidrefrigerant as it leaves the expansion valve. An outlet temperaturesensor 54 is positioned at the outlet end of the evaporator coil 44 todetect the temperature T_(o) of the superheated gaseous refrigerant asit leaves the evaporator coil 44.

Additional temperature sensor units are provided for system controlfunctions which include both the start-up condition and certain failsafe conditions for system safety. For example, a temperature sensor 36is positioned proximal the solenoid expansion valve 38 on the upstreamside thereof to monitor the temperature of the liquid refrigerant as itenters the expansion valve 38. Sensor 36 is functionally obtained toprovide control for low ambient start up conditions for the system.

An air temperature sensor 12 is positioned in the return air from thecooled unit 14 to determine the amount of cooling provided. Such airtemperature detection is applied to the expansion valve control circuit10 to effect temperature type control when the air temperature exceeds alow temperature setting. In other words, if the return air from the unitbeing cooled drops below a preset desired limit, the expansion valvecontrol circuit 10 operates to throttle down the refrigerant flow topermit the temperature within the unit cooled to rise above this lowerthreshhold value. When the temperature is again within an acceptablerange, the control circuit 10 is permitted to continue operations at thepoint where the circuit was operating when the temperature controlledshutdown function was initiated.

Turning now to FIG. 2, there is illustrated a control response curve fora typical prior-art expansion valve as used in a closed vapor cyclerefrigeration system. Typically, such control response presented alinear relationship between the superheat of the gaseous vapor andorifice opening size through which the liquid coolant flows, expressedas a percentage of full open fluid flow through the expansion valve 38orifice. In such a system, for example, the demand for fluid refrigerantinto the evaporator coil 44 to achieve a desired temperature coolingcould fluctuate from as little as 30% of maximum capcity, representing ademand condition A, to 60% of maximum at a second demand condition B.For the lower demand condition A, a set point A control norm wouldobtain where short term variations in demand about the set point A wouldoccur in order to maintain, on the average, the particular flow ratethrough the expansion valve 38 representing a 30% opening condition. Asdemand increases, the set point shifts in response to the increase indemand.

For the 60% flow capacity through the expansion valve 38, a second setpoint B would obtain. Short term demand variations about the set point Bwould obtain in a similar manner as occurs for any demand condition forthe system, including set point A. However, for the control responserelationship as illustrated in FIG. 2, a unit change in superheat aboutset point A will result in some change in the flow rate about the 30%position. Because of the linear relationship, the same unit change insuperheat about set point B for the 60% demand condition will producethe identical change in flow rate. Expressed as a percentage change, thechange in flow rate for demand condition A in response to the unitchange in superheat will be substantially greater than the percentagechange that same flow rate change represents if it were to occur aboutthe set point B. In other words, at higher flow rate demand conditions,a given unit change in superheat does not produce as great a percentagechange in the flow rate. An air conditioning system having an expansionvalve with the control response as shown in FIG. 2 cannot respond tochanging superheat conditions at higher demand flow rates with the samesensitivity that the system responds to at the lower flow rate.Accordingly, at higher flow rates, a greater change in the superheat isrequired in order to effect the same percentage change in the flow rateas occurs at lower rates. In accordance with the present invention,these and other limitations of the prior-art control valves have beeneliminated.

Turning now to FIG. 3A, there is graphically illustrates the controlresponse curve in accordance with the present invention for the closedloop control system as illustrated in FIG. 1. Not only has the presentinvention eliminated a shifting of the set points in response to achange in the demand rate of the refrigerant fluid through theevaporator 44, but it provides for a constant percentage change in theflow rate for a given unit change in superheat regardless of the flowrate condition or demand condition on the air conditioning system.

In accordance with the present invention, the control response curve forthe expansion valve 38 is not linear, but is representated as anexponential function. The expression for the control response curve maybe represented as a single exponential term whose exponent is a functionof the gaseous vapor superheat (T_(sh)), and includes a second DC offsetterm which is a function of the integral of the difference between thedesired operating set point superheat condition (T_(sp)) and theinstantaneous superheat which is occurring within the system (T_(sh)).With this DC term, if the superheat deviates from the set point for anyappreciable time, the response control function curve is, in effect,shifted by the integral term to the left in FIG. 3A. The controlresponse curve is shifted an amount necessary to bring the system to acondition in which the system responds to a unit change in superheatwith the same percentage change in flow rate regardless of what flowrate condition is present through the expansion valve 38.

In this manner, a single set point operating condition for the superheatis obtained for all flow rate demands through the expansion valve 38.Thus, a one unit change in superheat at a 30% demand flow conditionproduces a change in flow rate which, when expressed as a percentagechange, is the same as the percentage change in flow rate experienced ata higher demand condition for the same given one unit change insuperheat. This constant percentage change is illustrated in FIG. 3A,and results from the exponential nature of the response controlfunction.

The integral offset term in the expression for the control responsecurve modifies the position of the curve in FIG. 3 so that, in effect, achange in demand moves the operating point up and down the controlresponse curve. In other words, as the demand condition changes, thesystem operates at a different point on the response control functioncurve, such as the curve for operating condition A, which has beenshifted to the curve for operating condition B maintaining the singleset point operating point.

While the preferred embodiment of the present invention uses theexpedential relationship between the superheat and the percent of fullopen capacity for the expansion valve 38, it is possible to utilize thelinear relationship which is typically found in the prior-art controlsystems (See FIG. 2) to attain some of the advantages of the presentinvention, such as a single set point operating condition. However forsuch a linear relationship, some of the disadvantages present in theprior art will still be present in this alternate embodiment. That is,the given one unit change in superheat will produce a differentpercentage change in the flow rate as a function of the particulardemand condition which is occurring, but as below disclosed, otheradvantages for this embodiment can also be obtained.

Referring now to FIG. 4 there is shown a functional block diagram of theexpansion valve control circuit 10 illustrated in FIG. 1. Controlcircuit 10 functions to implement the control response curves andoperations as illustrated in FIG. 3A. Referring now to both FIGS. 1 and4, the outputs from inlet sensor 52 and outlet sensor 54 are shown inFIG. 4 inputted to a differential amplifier 18. Differential amplifier18 generates an output signal which is a function of the difference inthe temperature signals (T_(o) -T_(i)) received from these two sensorsto produce a signal representative of a change in the superheatcondition for the gaseous vapor. The output from amplifier 18 isinputted into an integrator control circuit 20 which, in turn, producestwo outputs control signals. A first output signal from control circuit20 is applied to one input of a second amplifier 24, while a secondoutput signal is inputted to an integrator 22. The output from theintegrator 22 is applied as an offset voltage to a second input of thesecond amplifier 24. Amplifier 24 produces an output control signalwhich is applied to a function generator 26. For the preferredembodiment of the present invention, function generator 26 is avoltage-to-pulsewidth converter, i.e., a pulsewidth modulator. Theoutput from the pulsewidth modulator 26 ultimately is applied to thesolenoid valve 38 to control the desired flow condition through theexpansion valve 38.

The integrator control circuit 20, integrator 22, amplifier 24, andvoltage-to-pulsewidth converter 26 each cooperate together to implementthe exponential conrol response curves as shown in FIG. 3A. Primarily,the exponential term for the control response curve is achieved from thetransfer function of the voltage-to-pulsewidth converter 26. The inputcontrol voltage to the voltage-to-pulsewidth converter 26 outputted byamplifier 24 is controlled by the integrator control circuit 20 andintegrator 22 to generate the integral component of the exponentialexponent for the control response curve. That is, the voltage on theoutput of the differential amplifier 18 is a voltage representing achange in the instantaneous superheat for the coolant gas at the outletside of evaporator 44. If a set point superheat operating condition of,for example, 4° F. superheat has been selected, the voltage on theoutput of amplifier 18 would represent the change in superheat from thisset point, e.g., 4° F.-T_(sh).

This superheat change voltage is applied to the integrator 22 and isintegrated over time to produce an output DC offset voltage foramplifier 24. The integrator control circuit 20 also applies the outputof the differential amplifier 18 directly to the input of amplifier 24.This direct connection of the output of the differential amplifier 18 tothe amplifier 24 enables the expansion valve control circuit 10 torespond instantaneously to short term variations in the superheat aboutthe set point. Only long term deviations from the set point superheatare reflected as control changes in the output of amplifier 24 as aresult of the integration process through the integrator 22. These longterm steady-state changes in the superheat from the set point representa changed demand condition which require a shifting of the controlresponse curve. As previously mentioned, this integration processeffectively shifts the response control curve in a direction to enablethe air conditioning system to effectively operate at a single superheatset point condition regardless of the demand conditions through theexpansion valve 38.

As described below and as shown in FIG. 5B, the integrator 22 functionis performed solely by analog circuits in the nature of an operationamplifier having associated resistors and capacitors connected in awell-known integrator configuration. While an analog embodiment of anintegrator is disclosed, other equivalent embodiments may be used. Forexample, a digital circuit implementation 6 of an integrator (shown inFIG. 4 in dashed lines) could just as easily be substituted for theanalog embodiment shown. A well-known digital circuit for an integratorconsists of an up/down counter 4 whose count represents the integrationvalue of an input voltage.

A count direction control signal is generated as a function of thedifference between a set point superheat and the operating superheat ofthe system. If the operating superheat is above the set point by somepredetermined amount, the count direction signal will permit the countin the up/down counter 4 to be increased by one count at a clock timedetermined from clock 3. If below the set point, the count is decreasedone count at the clock time. The period of the clocking signaldetermines the integration time constant for the digital integrator.

To obtain an analog voltage as a function of the count, adigital-to-analog (D/A) converter 5 is provided. The output from the D/A5 can be further processed to obtain the offset voltage needed byamplifier 24 to adjust the response control curve to a changed flow ratecondition.

Still referring to FIG. 4, the output from the voltage-to-pulsewidthconverter 26 is applied to the input of a control switch 17, which inturn applies the output from converter 26 to the input of the solenoiddriver 19 when the control switch 17 is closed. The solenoid driver 19output is applied directly to the solenoid of the solenoid valve 38 toelectrically actuate the setting of the flow condition through thevalve.

Additionally referring now to FIG. 6, there is shown a timing diagramfor the output voltage waveform for the voltage-to-pulsewidth converter26 as shown in FIG. 4. The timing waveform illustrated in FIG. 6 areintended to represent the two operating conditions A and B as shown inFIG. 3A. The frequency of the pulse train shown in FIG. 6 is constantregardless of system flow rate demand conditions, and for the preferredembodiment, this frequency is set for a period of seven seconds. Theduty cycle of each period of the waveform is controlled to effectuate adesired flow condition through the expansion valve 38. When the waveformin FIG. 6 is in a first logic state, a full open flow condition throughthe solenoid valve 38 is obtained. The opposite logic state produces afully closed orifice permitting no flow of liquid refrigerant throughthe valve. By varying the duty cycle of the pulsewidth modulatedwaveform, the average flow rate through the expansion valve 38 can beprecisely controlled. The frequency of the pulsed waveform is selectedto be high enough so that the slow response time of evaporator 44effectively filters out the flow--no flow pulsations of the fluid, butlow enough that the solenoid can respond to the opening and closingcommands to the valve's orifice.

One of the significant disadvantages of prior art expansion valves,whether mechanical or electromechanical, is the phenomenon known ashysteresis. Hysteresis results in an error in the exact orifice openingsize in response to a given control signal. For a typical prior artmechanical expansion valve, a 6° F. superheat hysteresis error is notuncommon. This superheat hysteresis error means that a change in thesuperheat conditions must exceed 6° F. before any change in the flowcondition through the expansion valve 38 will be affected.

In accordance with the present invention, it is possible to eliminatethis hysteresis effect. Hysteresis errors are eliminated by theoscillatory energization of the solenoid valve 38 to effectuate movementof the orifice closing mechanism between a first position represented bya maximum control voltage to a second position represented by a minimumcontrol voltage to the solenoid valve 38. If the control voltage to thesolenoid valve 38 is caused to oscillate at a frequency within theresponse range of the solenoid valve 38 and greater in magnitude thanthe hysteresis error, the hysteresis errors are averaged out so that, onthe average, the position of the orifice closing mechanism will be inthe desired position to achieve the desired flow rate therethrough. Inother words, if the control signal which effects movement of the orificeclosing mechanism is greater than the hysteresis error band and producesmovement of the orifice closing mechanism, the long term averaging ofthe orifice opening will be equal to the size predicted by the averagevalue of the control signal, i.e., the desired average flow rate wouldbe represented by the average value of the solenoid control signal.

The pulsed waveform illustrated in FIG. 6 illustrates a digital squarewave control signal to the solenoid valve 38 in which the orificeclosing mechanism is moved from a first position (logic state onerepresented by a maximum control voltage) in which the orifice is wideopen to a second position (logic state two represented by the minimumcontrol voltage) in which the orifice is fully closed. For the twocontrol voltage extremes illustrated for the waveform in FIG. 6, thecontrol signal is represented as an oscillatory waveform which obtains amovement from a fully open to a fully closed position.

It will be obvious to a person of ordinary skill that rather thancausing movement of the orifice closing mechanism to move from a fullyopen to a fully closed position, it is possible to have the controlsignal have a smaller amplitude as well as be characterized by either asinusoidal waveform or a squarewave waveform superimposed on a DCcomponent to achieve the oscillatory movement of the orifice closingmechanism between two positions about some average desired openposition. In other words, movement does not have to be from a fully opento a fully closed position in order to eliminate the hysteresis erroreffect which is inherent in the mechanical design of solenoid expansionvalves. Thus, in accordance with the principals of the presentinvention, it is possible to achieve a very accurate control of theorifice opening, on the average, by providing an oscillatory excitationto the solenoid control valve 38.

Even though the waveform illustrated in FIG. 6 for the control signal tothe solenoid valve 38 represents a maximum movement of the orificeclosing mechanism, the averaging effect of the hysteresis error is alsoobtained. The duty cycle of the pulsewidth modulated waveform determinesthe average flow through the orifice, which could be represented as anaverage position for the closing mechanism to achieve that desiredaverage flow rate. Thus, for operating condition A, the duty cycle ofthe time the orifice is fully open to the time that it is fully closedproduces one flow rate while the duty cycle for operating condition Bshows an open time which is greater thereby representing a higher demandflow condition through the solenoid valve 38.

Still referring to FIG. 6, it should be noted that the pulsewidthmodulated waveform shown therein is represented by leading and trailingedges which are sloping in nature as opposed to an instantaneous voltagechange characteristic of pulsewidth modulated waveforms. One of thesignificant problems which occurs in a close vapor cycle refrigerationsystem when a valve, such as the electrically-actuated solenoidexpansion valve 38 of the present invention, is abruptly opened orclosed to the fluid flow therethrough is a pressure shockwave phenomenonwhich is generated in a liquid refrigerant. This pressure shockwaveoccurs when the time rate of change in pressure is affected in theexpansion valve 38, i.e., the dp/dt term is high. Prior-art solenoidvalves are especially bad at producing this shockwave because of thenonlinear forces which act on the orifice closing mechanism at themoment of opening and closing to produce rapid movement, andaccordingly, large dp/dt values.

The effects of this pressure shockwave on the system are severe,especially in view of the large number of openings and closings of thesolenoid valve which must occur over the life of the valve. It has beenfound that such pressure shockwaves eventually render the airconditioning system inoperative due to damage to the various elementswhich together comprise the closed vapor loop system.

The present invention has solved this problem by controlling the dp/dtcondition in going from full flow to zero flow by ramping up and downthe control signal to the solenoid valve 38 in a controlled manner. Thisslowing down of the speed with which openings and closings occurminimizes the time rate of change of the pressure condition at theorifice of the valve 38. The opening time represented by the rampcondition on the pulsewidth modulated waveform controllably moves theorifice closing mechanism between the two positions representing thefull open and fully closed position. In this manner, the undesirablepressure shockwave in the refrigerant coolant is essentially eliminated.

In addition to the controlling of the openings and closings of valve 38by the ramp time of the pulsewidth modulated waveform from converter 26to minimize pressure impulses in the system, some measure of filteringof these impulses can be achieved by the use of line restrictions aheadof and behind expansion valve 38. For example, FIG. 1 illustrates theserestrictions as line restrictions 71 and 73, respectively. (Suchrestrictions could just as easily be incorporated into expansion valve38.) Two types of restrictions have been found to improve the filteringof these impulses, one type which is a simple restriction in thediameter of the line to a more complex type which monitors the flowtherethrough and modulates the restriction as a function of the pressuredrop thereacross to obtain a constant flow rate. This latter constantflow control device tends to eliminate variations in flow rate due topressure changes across the restriction.

Referring once again to FIG. 4, there is shown additional controlfunctions which regulate extreme conditions encountered during start upand normal operations for the refrigeration system shown in FIG. 1. Asshown in FIG. 1, a temperature sensor 70 is placed on the liquid line 36for purposes of determining a low ambient start condition. This liquidtemperature sensor 70 is inputted to a comparator 23 which compares thetemperature difference between the temperature of the upstream siderefrigerant coolant to the temperature on the downstream side from inletsensor 52. If the two temperatures are essentially the same, a LOWAMBIENT START control signal is generated to actuate the solenoid closeswitch 13. When the temperatures on the upstream and downstream side ofthe solenoid expansion valve are essentially the same, as would bepresent for a low ambient start condition, gas is essentially present inthe liquid line 36 and no expansion is occurring across the expansionvalve 38.

The output from the solenoid close switch 13 is applied as a clampingvoltage to the solenoid driver 19 thereby to actuate the driver to openthe solenoid valve 38 to its full open position. If the LOW AMBIENTSTART signal is active, the valve 38 is held open permitting fluid toflow when such fluid is present.

The LOW AMBIENT START control signal is continuously generated to keepthe solenoid valve 38 fully open as the system starts up. Eventually,liquid refrigerant will enter the solenoid valve 38 and experience someexpansion thereby to create a temperature differential across thesolenoid valve representing a condition at which the control systemshould then begin to operate. Under this condition, the comparator 28removes the LOW AMBIENT START signal thereby removing the clamp voltageto the solenoid valve 19. To insure that the system will start on powerup (0° superheat) even if a low ambient start condition is not present,the circuit of the valve control circuit 10 are designed to produce aminimum pulsed duty cycle control signal to the expansion valve 38 of10%. With this minimum duty cycle, the expansion valve 38 will at leastbe open some of the time to permit fluid to enter the evaporator coil44.

During normal operations, several additional system conditions can occurwhich require gross control to the solenoid expansion valve 38. Forexample, a flooding condition can occur in which the temperature sensorat the outlet side of the evaporator coil 44 senses that a superheatedgas is not coming out of the evaporator coil, liquid refrigerant is alsopresent. Liquid in the suction side of compressor 30 is a dangerousoperating condition for the compressor. This is referred to as a FLOODcondition, and is indicated when the temperature differential betweenT_(o) and T_(i) is essentially zero.

Comparator 19 responds to the temperature differential between the inletsensor 52 and the outlet sensor 54 to generate a FLOOD control signalwhen T_(o) -T_(i) is zero. This signal is applied as one input to NORgate 25. The output from NOR gate 25 is the signal FAST SHUT DOWN, whichis inputted to the integrator control circuit 20 to effectively throttledown the control operation to the expansion valve 38 to lessen theamount of liquid refrigerant being injected into the evaporator coil 44.This throttling operation is achieved by injecting a voltage into theintegrator 22 which represents a lower demand condition therebyresulting in a decrease in the flow rate through the expansion valve 38.

In a similar manner, it is desired to limit the lower operatingtemperature for the refrigerant coolant through the compressor 30 inorder to avoid damage to the compressor. The inlet temperature frominlet sensor 52 is applied to a comparator 21 which compares the inlettemperature to a maximum lower operating temperature setting alsoapplied to the comparator 21. When the inlet temperature drops below themaximum lower operating temperature setting, the comparator 21 outputs alogic control signal to the second input of the NOR gate 25 to alsogenerate the signal FAST SHUT DOWN, and thereby obtain the samethrottling operation as previously described.

Finally, an air temperature sensor 12 is provided to sense thetemperature of the return air, or the ambient air conditions at anyplace within the system that is desirable to be controlled. This sensoris inputted to a temperature control circuit 29 which generates anoutput control signal when the refrigerated air temperature exceeds apreset threshhold. The output from the temperature control circuit 29 isapplied as a control signal to control the condition of the controlswitch 17 and as the input voltage applied through the solenoid closedswitch 13 when a LOW AMBIENT START condition is present. When monitoringthe return air temperature, if the ambient air temperature exceeds alower threshhold value, the control switch 17 is opened removing thesignal from the voltage-to-pulsewidth converter 26 from the input of thesolenoid driver 19. Removing the converter 26 output closes the solenoidexpansion valve 38 and prohibits further flow of the liquid refrigerantinto the expansion coil 44. At the same time, the output of the controlswitch 17 causes the NOR gate 15 to generate the signal INTEGRATION LOCKwhich is applied to the integrator control circuit 20. INTEGRATOR LOCKprevents the integrator 22 from changing the offset voltage on itsoutput from the condition which is present at the time control switch 17is opened. This in effect takes a snapshot picture of the conditions ofthe control circuits at the moment that the ambient temperature signalcaused the system to be shut down so that when the ambient air conditionis again in an acceptable range, the system can pick up from the pointat which it was operating when the temperature exceeded the threshhold.

Turning now to FIGS. 5A, 5B and 5C, there is illustrated a detailedcircuit diagram of the expansion valve control circuit 10 as illustratedin FIG. 4, when FIG. 5C is placed to the right of FIG. 5B and FIG. 5A isplaced to the left of FIG. 5B. Each of the functional blocks illustratedin FIG. 4 are shown in FIG. 5. The operations of the integrated circuitsillustrated in FIGS. 5A, 5B and 5C are well known to those of ordinaryskill in the art, and accordingly, a detailed description of theiroperation will not be provided. However, additional features notpreviously discussed are illustrated in the detailed circuit diagram.For example, if the difference between the inlet temperature and outlettemperature exceeds a maximum threshold, the integrator control circuit20 itself produces a gross system correction to bring the temperaturedifferential back within an acceptable operating range. Resistor R4 inseries with Zenor diode D20 responds to the voltage representing thedifference between the set point and the instantaneous superheattemperature for the outlet temperature. If that voltage exceeds somemaximum temperature differential, for example 32° F. superheat, theZenor diode D20 will conduct and apply a voltage to the input of theintegrator 22 to produce a rapid change in the offset voltage into theamplifier 24. This rapid change in offset throttles the system down to asituation represented by a low demand condition thereby causing thetemperature differential between the inlet and outlet of the expansioncoil 44 to decrease. Additionally, the back-to-back diodes D8 and D9limit the normal voltage range into the control circuit to thereby limitthe maximum detectable change in superheat to which the control circuitwill respond. Changes in superheat which exceed the limiting voltage ofthe diodes but are less than the maximum superheat which trips Zenerdiode D20 result in a constant change in control as determined by thelimiting action of diodes D8 and D9. Changes in superheat which arebelow the limiting range of diodes D8 and D9 effect a change in controlor a function of the magnitude of the change.

As shown in both FIGS. 4 and 5C, a light emitting diode LD1 is connectedto the output of the solenoid driver 19 thereby to indicate the amountof time that the solenoid valve 38 is open. As the demand conditionincreases, the intensity of light emitted by the diode LD1 increasesthereby indicating an increase in the demand of the system. Anindication by LD1 that the solenoid valve is operating in a high demandcapacity is indicative of a low refrigerant condition within the system.As shown in FIG. 4, an alternate low refrigerant detector could beincorporated into the circuits for the expansion valve control circuit10 whereby the output from the amplifier 24 would be compared with apreset level condition to indicate a condition in which the duty cyclefor the pulsewidth modulated signal is higher than some preset conditionfor some predetermined time interval. The output from the comparison ofthese signals could be used to generate an alarm signal indicating thelack of refrigerant condition. This feature could also be accomplishedby monitoring the superheat signal, and if it remains above apredetermined value for a given amount of time, an alarm is indicated.

Turning now to FIG. 7, a preferred flow control valve in accordance withthe present invention is shown in vertical cross-section. Valve body 90defines generally therein a chamber 92 connected to an inlet port 94 andan outlet port 96. The inlet port is defined by a connection 98 havingan externally threaded end 100, which threads are accommodated by theinternal threads in the bottom of the valve body. A protection cap 102is shown screwed over the external threads of the exposed end of theinlet port, which cap would be removed when the port is connected to asuitable line.

Outlet port 96 is generally provided by a valve seat part 104 which isjoined into the side of chamber 92 by being screwed into suitableaccommodating threads therein. The outlet port by being screwed into theside of the chamber, is therefore normal or perpendicular to the inletport screwed into the bottom of the chamber and in line with the plungeraction described below. A connection fitting 106 is screwed oversuitable accommodating threads in seat 104 to hold the seat in position.As with the inlet port, a suitable protection cap 108 is provided overthe external threads of the inlet port assembly just described toprotect it during shipping. The cap is removed in order to provideconnection to a suitable line.

Operating within chamber 92 just defined is a valve closure 110 whichoperates in a shear relationship to valve seat 104. That is, closure 110operates by sliding along the face of the seat to open and close theentry to the outlet. Valve closure 110 is biased forward or toward thevalve seat by a suitable biasing spring 112 carried in the valve closureblock. The tension on spring 92 can be adjusted by a suitable screwwhich is entered through a suitable opening on the opposite side of thevalve closure block from the valve closure itself.

A suitable plunger 114 operates within valve body 90 at the upper endthereof so as to operate within an upward extending portion of the bodywhich surrounds the plunger and a suitable slide assembly sleeve 116which is located adjacent to the internal periphery surface of theupwardly extending portion of the body and is, hence, around andadjacent the plunger itself. The material of this slide assembly ispreferably Teflon or some other suitable material with low frictionproperties to permit easy movement of the plunger within the slideassembly.

The bottom portion of plunger 114 is provided with suitable internalthreads for accommodating the external threads of an inward extension ofthe valve closure block in a fixedly tight arrangement therewith. Thatis, the plunger carries the closure block as it moves up and down inoperation.

It should be further noted that the bottom end 118 of the plunger isjust slightly below the upward shoulder 120 of chamber 92, shoulder 120defining the lowest part of the valve body which provides magnetic fluxto the plunger during its operation. The bottom end of the plunger isalso conically tapered at surface 119. The magnetic operation of theplunger is described more fully hereinafter.

The upward end 122 of plunger 114 is conically shaped at a preferredangle between 30° and 60°. The valve block portion which is opposed toconical surface 122 has an internal compatible surface 124 so that thereis a magnetic gap between surfaces 122 and 124 which is provided duringoperation of the plunger. The upward extension of the valve block justdescribed is fixedly in contact with the solenoid housing to bedescribed hereinafter. As such, it becomes an operable part of thishousing.

A post 126 is provided in the upper end of the plunger, which is cappedby a suitable spinner 128 onto which a suitable biasing spring 130operates. The spring is accommodated within a chamber in the housingportion which is located over the post 126 and spinner 128 justdescribed. This chamber is internally threaded to accommodate anexternally threaded plug 132 which has a depending portion 134 fittinginto spring 130 and which determines the amount of biasing forceprovided by spring 130 in a downward direction for the plunger. Plug 132can be conveniently sealed in place by solder 136 once the propertension bias adjustment has been made to the spring.

Valve body 90 is of ferromagnetic material, as is plunger 114. Also, thesolenoid housing described is of ferromagnetic material.

Now referring to FIG. 8, the valve which has just been described isshown with a solenoid assembly generally referred to by referencenumeral 138 surrounding the upper end of the housing portion of thevalve body which has just been described. A coil 140 provides magneticflux for operating the plunger through signals provided by electricleads 142. In the preferred electronics control circuit described above,such signals are in the form of duty cycle oscillations, the leading andtrailing edges of which are ramp shaped. The solenoid housing 144 isprovided with suitable bottom and top plates and is held in position bya cap 146 which is screwed onto external threads of the top portion ofthe valve body housing.

Now with respect to FIG. 7, magnetism provided by the solenoid coilcauses the plunger to move upwardly to reduce the size of the gapbetween surfaces 122 and 124 which have previously been described,thereby opening the outlet port at valve seat 104 and closure 110. Whenthe magnetism decreases, the bias spring causes the valve closure toshut vis-a-vis the valve seat. The plunger has a linear response to theaverage applied signal to the coils until the bottom end of the plunger118 rises in its movement above shoulder 120. At this time, magneticflux is provided inclose proximity to the end of the plunger andtherefore causes an attraction of the plunger in opposition to themagnetism which is provided by the gap at the upper end thereof. Conicalshaping of surface 119 provides fine tuning of the magnetics byproviding a nonuniform gap between the solenoid plunger and the housing.Hence, the plunger linear movement in response to applied magnetic fluxno longer is obtained, but instead the plunger slows down and eventuallystops before the gap at the upper end is reduced to surface touching.That is, the upper end never touches the valve body housing. It shouldbe noted that the lower end of plunger 114 is tapered or otherwiseshaped and provides gap characteristics that are necessary to slow downthe plunger operation when it is operating at the upper limits of theplunger movement. This helps prevent unnecessary shocks from beingimparted to the fluid through the valve. The shear operation of closure110 with respect to seat 104 also assists in minimizing shocks to thefluid through the valve.

Inlet port connection 98 is preferably in line with the axis of theplunger, but it can be offset or at an angle to the chamber other thanin line, if desired. Alternatively, the sliding valve seat and valveclosure action can be in conjunction with the inlet port, rather thanthe outlet port. In such arrangement, the inlet port would be connectedto the side of the chamber.

In operation, the valve described and shown will not only slow down asit approaches the upper limit of plunger movement as it fully opens thevalve outlet port, it also moves slowly in initiating shutting actionuntil the gap at the lower end disappears.

As previously mentioned, flow restrictions can be provided in the systemto help regulate the problems presented by the pulsed opening andclosing of the solenoid expansion valve 38. One such type of restrictionis commonly called a distributor which is positioned downstream of theexpansion valve at the inlet side of the evaporator 44. In fact, adistributor is nothing more than a plurality of fluid lines 40connecting the output side of expansion valve 44 to the input side ofevaporator 44. With prior-art expansion valves, "flashing" normallyoccurs at the output side of the expansion valve because of the pressuredrop thereacross. Flashing is a term of art which describes the presenceof gas bubbles in the liquid coolant as it exits the expansion valve.With the pulsing operation of expansion valve 38 in accordance with thepresent invention, this pressure drop across the valve is effectivelyminimized since the valve is either fully open or fully closed.Accordingly, flashing is minimized, and as a result, the fluid flow intoeach of the distributor tubes 40 is essentially equal, i.e., the fluidis equally distributed into each tube. Better fluid flow results in lesspressure drop across the restriction and better flow controltherethrough.

It has been noted that the oscillating of the valve closure mechanismaverages out and thereby eliminates hysteresis errors in the control ofthe expansion valve. Heat motors connected for powering the valveplunger instead of a magnetic solenoid could also be used, but thesetype of motors also suffer from hysteresis effects. Oscillating suchmotors about a controlled value would thereby average out the hysteresiserrors in such a heat motor system.

In an additional aspect of the invention, the solenoid valve of thepresent invention could be controlled to effect communications throughthe components of an air conditioning system itself. Movement of theorifice closing mechanism to increase or decrease the orifice openingproduces a pressure variation in the fluid coolant flowing therethrough.If an information coded signal having a frequency near the upper rangeof response of the solenoid valve is superimposed onto the normalcontrol signal to the valve, it is possible to inject into the fluid asa pressure signal the information carrying signal. Appropriate pressuredetectors could be placed in the system to receive the transmittedinformation. In this manner, control information, for example, could betransmitted from one point to another point by the coolant fluid as thetransmission medium.

In accordance with the present invention, because of the large controlrange in the valve 38 it is possible to vary the operating parameters ofthe refrigerant system and still maintain the superheat. By allowingcondensing temperatures to fall in low ambient temperature conditions,the system can run at a much higher coefficient of performance thusproviding a great energy savings. This energy savings is reflected as areduction in the amount of energy required to operate the system.

In describing the invention, reference has been made to its preferredembodiment. However, those skilled in the art and familiar with thedisclosure of the invention may recognize additions, deletions,substitutions, or other modifications which would fall within the purvueof the invention as defined in the appended claims.

What is claimed is:
 1. In combination with a closed vapor cycle refrigeration system including in connected closed loop sequence, a compressor, a condenser, a solenoid actuated expansion valve and an expansion evaporator, improved electronic expansion valve apparatus, comprising:(a) a means for sensing the superheat of the expansion evaporator and producing an electrical signal level indicative thereof; (b) a converting means connected to said sensing means and producing from the electrical signal level an on-off modulated signal whose duty cycle is representative of the required flow rate of liquid refrigerant through said valve; and (c) a means responsive to said on-off modulated signal for slowly opening and closing said valve without imparting any substantial pressure shockwaves to the liquid refrigerant, said expansion valve being modulated operated by said converting means and said means for slowly opening and closing said valve from an open position to a closed position for each cycle of the on-off modulated signal to produce an average refrigerant flow rate therethrough in accordance with the duty cycle of said on-off modulated signal.
 2. An electrically-operated solenoid expansion valve for controlling refrigerant flow into the evaporator coil of a closed vapor cycle refrigeration system, comprising:(a) a valve closure means having a closure for opening and closing fluid flow from the condenser coil of the refrigeration system to said evaporator coil, (b) an electrical control means for alternately operating said valve closure means opened and closed over a period of time to maintain the fluid in said evaporator coil substantially in its liquid state, and (c) a means for slowly opening and closing said closure means so as not to impart a substantial pressure shockwave into said fluid with each opening and closing of said valve.
 3. An electrically operated expansion valve in accordance with claim 2, wherein said valve closure means is a shear acting closure mechanism.
 4. An electrically operated expansion valve in accordance with claim 2, wherein said valve closure means includes magnetic means for operating said closure in linear fashion through at least a substantial range of closure movement.
 5. A solenoid actuated expansion valve for use in a refrigeration system having in a closed loop connection a compressor having an inlet and an outlet end, a condenser connected to the outlet end of said compressor and responsive to a high pressure gaseous phase recirculating refrigerant for condensing the refrigerant from its gaseous to its liquid phase, an evaporator having an inlet and an outlet end and connected to said condenser and to the inlet end of said compressor, an expansion valve having an inlet and an outlet end and connected between the outlet end of said condenser and the inlet end of said evaporator, said condenser delivering high pressure liquid refrigerant to the inlet end of said expansion valve, said refrigerant expanding as it flows through said expansion valve, said valve including an on-off modulator responsive to the superheat of said refrigerant in said evaporator for generating a variable duty cycle on-off modulated solenoid control signal, and a means responsive to said on-off modulated signal for slowly opening and closing said valve without imparting any substantial pressure shockwaves to the liquid refrigerant, said solenoid control signal alternately energizing and deenergizing said solenoid for each cycle of said control signal to cycle said valve from a first flow rate position to a second flow rate position thereby to obtain an average flow rate of refrigerant through the valve which results in a desired superheat for the refrigerant.
 6. The valve of claim 5 wherein said first flow rate position results in a fully open maximum flow rate therethrough, and said second flow rate position results in a fully closed zero flow rate therethrough, said average flow rate being determined by the duty cycle of said solenoid control signal.
 7. The valve of claim 5 wherein said means for slowly opening and closing said valve further includes a closure rate control means controlling the rate of change in flow rate positions of said valve between said first and said second flow rate positions, and vice versa, thereby to minimize any pressure shockwaves in said refrigerant on opening or closing of said valve.
 8. The valve of claim 6 wherein said means for slowly opening and closing said valve further includes a closure rate control means for controlling the rate of change in flow rate positions of said valve between said fully open and said fully closed positions, and vice versa, thereby to minimize any pressure shockwaves in said liquid refrigerant on opening or closing of said valve.
 9. The valve of claims 7 or 8 wherein said closure rate control means is an electronic circuit which decreases the slope of the pulse edges in the on-off solenoid control signal.
 10. The valve of claims 6, 7 or 8 wherein said valve further includes:(a) a superheat sensing means coupled to said evaporator for sensing the instantaneous superheat of the refrigerant in said evaporator; (b) a superheat setting means for selecting a desired superheat operating set point for said refrigerant; and (c) a differential amplifier means responsive to the sensed instantaneous superheat and said desired superheat operating set point for generating a duty cycle control signal to said on-off modulator as a function of the difference between said sensed and said superhet operating set point.
 11. The valve of claim 10 wherein said superheat sensing means includes:(a) an inlet temperature sensor for sensing the temperature of said refrigerant at the inlet end of said evaporator; and (b) an outlet temperature sensor for sensing the temperature of said refrigerant at the outlet end of said evaporator, the difference between said inlet and said output temperature representative of the instantaneous superheat of said refrigerant.
 12. The valve of claim 10 wherein said differential amplifier means includes:(a) a first differential amplifier responsive to the sensed superheat and the superheat operating set point for generating an operating point control signal representative of the difference between the sensed superheat and the superheat operating set point; (b) an integration means responsive to the operating point control signal for generating an operating point shift control signal as a function of the time average of the difference between the instantaneous sensed superheat and the superheat set point; and (c) a second differential amplifier responsive to the operating point control signal and the operating point shift control signal for generating the duty cycle control signal to said on-off modulator, the operating point shift control signal dynamically shifting the effective operating set point of the valve without changing the single superheat operating set point of said superheat setting means.
 13. The valve of claim 12 wherein said integration means comprises an analog amplifier integrator circuit.
 14. The valve of claim 12 wherein said integration means is a digital integrator comprising:(a) a clock for generating timing pulses; (b) a comparator circuit responsive to the operating point control signal and a threshhold voltage for generating an up/down control signal; (c) an up/down counter responsive to the timing pulses and the up/down control signal, said counter counting up when the operating point control signal is above the threshhold and counting down when the operating point control signal is below the threshhold; and (d) a digital-to-analog converter for converting the count in said counter into an analog voltage as the operating point shift control signal.
 15. The valve of claim 8 further including a driving amplifier circuit responsive to the output from said on-off modulator circuit for providing power driving signals to said solenoid.
 16. The valve of claims 7 or 8 further including a low ambient start-up means responsive to the presence of liquid refrigerant at the inlet end of said expansion valve for overriding normal operations of said valve and maintaining said expansion valve open for refrigerant flow therethrough when liquid refrigerant is not present, said low ambient start-up means returning control for normal operations of said valve when liquid refrigerant is present.
 17. The valve of claim 16 wherein said low ambient start-up means comprises:(a) a first temperature sensor coupled proximal to the inlet end of said expansion valve, said first sensor generating a voltage representative of the temperature of the liquid refrigerant from said condensor; (b) a second temperature sensor coupled proximal to the outlet end of said expansion valve, said second means generating a voltage representative of the temperature of the expanding liquid refrigerant into the inlet end of said evaporator; (c) a comparator circuit responsive to the voltages from said first and second temperature sensors for generating a low ambient start signal when the temperature differential between said first and second sensors is less than a temperature threshold; and (d) a clamping means responsive to the low ambient start signal for overriding normal control of said expansion valve and maintaining said expansion valve open for refrigerant flow therethrough until the temperature differential between said first and second sensors is greater than the temperature threshold indicating expansion of liquid refrigerant through said valve.
 18. The valve of claim 17 further including an ambient air temperature sensing means responsive to the ambient air temperature for disabling said low ambient start up means from controlling said expansion valve when said ambient air temperature is below a minimum temperature, said ambient temperatue sensing means disabling said valve at the operating conditions then present so that when the ambient air temperature exceeds the minimum temperature, said valve may resume normal operations with the conditions present when the air temperature last dropped below the minimum.
 19. The valve of claims 7 or 8 further including a flood detector means responsive to the outlet-to-inlet temperature differential across said evaporator coil for throttling down the flow rate through said expansion valve when the temperature differential is less than a temperature threshold, said flood detector means generating a throttling control signal representative of a lower heat load condition on said evaporator coil to said valve as long as the temperature differential is less than said temperature threshold, said valve resuming normal operation from the throttled-down condition when said temperature differential is greater than said temperature threshold.
 20. The valve of claim 19 wherein said flood detector means comprises:(a) a first temperature sensor responsive to the temperature of the refrigerant at the inlet end of said evaporator; (b) a second temperature sensor responsive to the temperature of the refrigerant at the outlet end of said evaporator; and (c) a first comparator means responsive to said first and second temperature sensors for generating a flood condition signal when the temperature differential between said first and second sensors is less than said temperature threshold, said flood condition control signal representing a reduced heat load demand condition on said evaporator and which is applied to said valve to effect throttling down of the flow of liquid refrigerant through said expansion valve, said comparator means returning normal control to said expansion valve when said temperature differential between said first and second sensors is greater than said temperature threshold.
 21. The valve of claims 19 or 20 further including a second comparator means responsive to the absolute temperature of said liquid refrigerant at said inlet end of said evaporator and to a maximum operating refrigerant temperature signal for outputting to said controller means a maximum operating temperature control signal to throttle down the flow of refrigerant through said expansion valve when the temperature of said liquid refrigerant at the inlet end of said evaporator coil is greater than the maximum operating refrigerant temperature, said comparator means returning normal control to said expansion valve when said evaporator inlet temperature is less than the maximum refrigerant temperature.
 22. The valve of claim 21 further including an OR gate responsive to the flood condition control signal and the maximum operating temperature control signal for outputting to said controller means a fast shut down control signal to effect throttling down of the flow through said expansion valve when either the flood condition control signal or the maximum temperature signal indicates, respectively, that a flood condition exists in said evaporator coil or that the absolute temperature of the liquid refrigerant into said evaporator coil is above a maximum limit.
 23. The valve of claims 7 or 8 further including a low refrigerant detection means responsive to the percent on time of said valve for generating a low refrigerant alarm signal when the percent on time of the valve exceeds a threshhold value for a period of time.
 24. The valve of claim 23 wherein said low refrigerant detection means comprises:(a) a comparator responsive to the input signal to said on-off modulator, such input signal representative of the duty cycle of the solenoid control signal and to a duty cycle threshhold signal; (b) a reset timer coupled to the output from said comparator for generating an output when the duty cycle of the solenoid control signal is above the threshhold value for a period of time determined by said timer; and (c) an alarm circuit responsive to the output from reset timer to indicate a low refrigerant condition in said system.
 25. In a refrigeration system having in a closed loop connection a compressor having an inlet and an outlet end, a condenser connected to the outlet end of said compressor and responsive to a high pressure gaseous phase recirculating refrigerant for condensing the refrigerant from its gaseous to its liquid phase, an evaporator having an inlet and an outlet end connected to said condenser and to the inlet end of said compressor, a solenoid actuated expansion valve having an inlet and an outlet end and connected between the outlet end of said condenser and the inlet end of said evaporator, said condenser delivering high pressure liquid refrigerant to the inlet end of said solenoid actuated expansion valve, said refrigerant expanding as it flows through said solenoid actuated expansion valve, and a controller circuit responsive to the superheat of the refrigerant in said evaporator for controlling the flow of refrigerant through said solenoid actuated expansion valve, said controller circuit including an on-off modulator for generating an on-off modulated solenoid control signal whose duty cycle varies as a function of the superheat, a means responsive to said on-off modulated signal for slowly opening and closing said valve without imparting any substantial pressure shockwaves to the liquid refrigerant, and a low refrigerant detection means responsive to the duty cycle of the on-off modulated solenoid control signal for generating a low refrigerant alarm signal when the duty cycle of the solenoid control signal exceeds an upper threshold value for a period of time.
 26. The valve of claim 25 wherein said low refrigerant detection means comprises:(a) a comparator responsive to the input signal to said on-off modulator, such input signal representative of the duty cycle of the solenoid control signal and to a duty cycle threshhold signal; (b) a reset timer coupled to the output from said comparator for generating an output when the duty cycle of the solenoid control signal is above the threshhold value for a period of time determined by said timer; and (c) an alarm circuit responsive to the output from reset timer to indicate a low refrigerant condition in said system.
 27. A solenoid actuated expansion valve for use in a refrigeration system having in a closed loop connection a compressor having an inlet and an outlet end, a condenser connected to the outlet end of said compressor and responsive to a high pressure gaseous phase recirculating refrigerant for condensing the refrigerant from its gaseous to its liquid phase, an evaporator having an inlet and an outlet end and connected to said condenser and to the inlet end of said compressor, an expansion valve having an inlet and an outlet end and connected between the outlet end of said condenser and the inlet end of said evaporator, said condenser delivering high pressure liquid refrigerant to the inlet end of said expansion valve, said refrigerant expanding as it flows through said expansion valve, said expansion valve having a movable orifice opening element responsive to a solenoid control signal for obtaining an orifice opening size, said valve having hysteresis errors in the position of said orifice opening element in response to said solenoid control signal, said valve including a means for generating a solenoid control signal having an oscillatory component whose frequency is within the response range for movement of said orifice opening element and which causes said orifice opening element to slowly move between a first position and a second position without imparting any substantial liquid shockwaves to the refrigerant, thereby eliminating said positional hysteresis errors in obtaining a desired orifice opening size.
 28. The valve of claim 27 wherein said solenoid control signal further includes an average value component representative of a desired orifice opening size, said orifice opening element responsive to said oscillatory component moving between said first and second positions about a desired average opening size.
 29. The valve of claim 27 wherein said oscillatory component is a square wave.
 30. The valve of claim 29 wherein said means for oscillating said element includes a means for generating an on-off modulated control signal whose duty cycle represents a desired average flow rate through the valve, said oscillating means responsive to the on-off control signal slowly moving said orifice opening element from a closed position to an open position for each cycle of said on-off modulated control signal without imparting any substantial liquid shockwaves to the refrigerant.
 31. The valve of claim 30 wherein the amplitude of said oscillatory component of said solenoid control signal is representative of the magnitude of the positional hysteresis error and causes no positional movement of said orifice opening element. 